with GROUP C1/5 resistance: (PHOTOSYSTEM II INHIBITORS)Inhibition of photosynthesis at photosystem II

MUTATION: PROLINE 197 to SERINE

Downy Brome (Cheatgrass) (Bromus tectorum) is a monocot plant in the poaceae family. A single amino acid substitution from Proline 197 to Serine has led to resistance to Photosystem II inhibitors as indicated in the table below.

Primisulfuron-resistant (AR and MR) and -susceptible (AS and MS) Bromus tectorum biotypes were collected from a Poa pratensis field at Athena, Oregon, and in research plots at Madras, Oregon. Studies were conducted to characterize the resistance of the B. tectorum biotypes. Whole plant bioassay and acetolactate synthase (ALS) enzyme assay revealed that the AR biotype was highly resistant to the sulfonylurea (SU) herbicides, primisulfuron and sulfosulfuron and to a sulfonylaminocarbonyltriazolinone (SCT) herbicide, propoxycarbazone-sodium. However, the AR biotype was not resistant to imazamox, an imidazolinone (IMI) herbicide. Results of the whole plant bioassay studies showed that the MR biotype was moderately resistant to all ALS inhibitors tested. However, there were no differences in ALS sensitivities between the MR and MS biotypes. The nucleotide and amino acid sequence analysis of the als gene demonstrated a single-point mutation from C to T, conferring the exchange of the amino acid proline to serine at position 197 in the AR biotype. However, this mutation was not found in the MR biotype. Results of this research indicate that: the resistance of the AR biotype to SU and SCT herbicides is based on an altered target site due to a single-point mutation; resistance in the MR biotype is not due to a target site mutation..